Stainless steels are classified into large families depending on their metallurgical structures, after a heat treatment. Martensitic ferritic, austenitic and austenoferritic stainless steels are known.
The latter family comprises steels which are generally rich in chromium and nickel, that is to say that they have respective chromium and nickel contents greater than 20% and greater than 4%. The structure of these steels, after treatment at a temperature of between 950.degree. C. and 1150.degree. C., consists of ferrite and of austenite in a proportion generally greater than 30% for both phases.
These steels have many practical advantages, in particular they have, in the annealed state, for example after being annealed at 1050.degree. C., mechanical properties, especially yield stress, which is much higher than ferritic or austenitic stainless steels in the annealed state. On the other hand, the ductility of these steels is of the same order of magnitude as that of ferritic steels and lower than that of austenitic steels.
One of the advantages of austenoferritic steels relates to weld properties. After a welding operation, the structure of these stainless steels, in the melt zone and in the heat-affected zone, remains highly polyphase in terms of ferrite and austenite, contrary to austenitic steels in which the weld remains mainly austenitic. This results in high mechanical properties of the welds, properties which are desirable when welded assemblies must withstand mechanical stresses in operation.
Finally, certain austenoferritic steels containing finely divided austenite may have a high plasticity called superplasticity during hot slow forming.
These austenoferritic steels also have drawbacks such as, for example, their high cost, because their composition has a high nickel content or because of manufacturing difficulties, especially those related to their high chromium content, such as, for example, the formation of an embrittling sigma phase or separation into an iron-rich ferrite and a chromium-rich ferrite with embrittlement of the steels during cooling after hot rolling.
Their ductility, measured by the tensile elongation at ambient temperature does not exceed 35%, which renders its processing, by drawing, forging or any other process, difficult.
Embrittlement also occurs during use of the steel at a temperature above 300.degree. C. when the temperature hold exceeds a few hours.